In a government decree on Sunday, Mr. Hun Sen said the waterlogged communes and villages that usually send their longboat teams to Phnom Penh for three days of races each year in November to celebrate the turning of the Tonle Sap River—and the 1 to 2 million people who come to watch them—ought to focus instead on recovering from the recent floods.

But with the city still locked in a political standoff over July’s disputed national election, and the opposition continuing to draw tens of thousands of supporters to their rallies, the CNRP has said Mr. Hun Sen’s explanation is a mere excuse.

CNRP chief whip Son Chhay said the prime minister was actually terrified of millions of people flowing into the city for the event.

“The government is scared as hell of them,” he added, referring to the rural residents who would likely flood Phnom Penh during the Water Festival, which had been scheduled for November 16, 17 and 18. “When people come together, it is a threat to this government.”

“I think it’s quite obvious. The decision is based on his concern over the opposition demonstrations,” he said. “Now I believe he is really concerned about that, to stop people coming to Phnom Penh while the political situation is not yet settled.”

Political analyst Kem Ley agreed.

“Every year at least 2 million people come and right now the majority of people support the CNRP, so if a lot of people come to Phnom Penh it is a big problem for the CPP,” he said.

While the CPP officially won a majority of the vote, the opposition alleges widespread fraud and says it was the real victor.

Mr. Ley added that the decision to cancel the event would likely prove unpopular with local vendors, many of whom count on the large crowds to pad their annual profits.

Mr. Chhay said the government was likely also worried that it might not be able to prevent a repeat of the last time Phnom Penh hosted the festival in 2010. On the last day of festivities that year, 353 people were trampled to death after organizers allowed too many revelers onto a bridge connecting Koh Pich to the mainland.

“The system is so corrupt hardly anyone was held accountable,” he said (in fact, no one was held accountable). “Unless you have a reliable system in place, you cannot manage [the crowds].”

The government canceled the festival last year to honor King Father Norodom Sihanouk, who died of heart failure at the age of 89 in October. The festival was also canceled in 2011 due to heavy flooding.

This year’s flooding has been less severe than in 2011, however, and less than in 2009, when the government went ahead with the festival.